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<H1>empty_queue(Queue)</H1>
tests whether the queue is empty
<H2>Description</H2>

    tests whether the queue is empty.  If the back of a queue were
    guaranteed to be a variable, we could have
<PRE>
 	empty_queue(Front-Back) :- var(Front).
</PRE>
    but I don't see why you shouldn't be able to treat difference
    lists as queues if you want to.
    

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